In the production of fluids such as hydrocarbons from a subterranean well, it may be desirable to selectively seal or plug the well at various locations. For example, in hydrocarbon (oil and/or gas) production wells, it may be necessary or desirable to seal off a lower hydrocarbon-producing formation during the extraction of hydrocarbons from an upper hydrocarbon-producing formation. In other applications, it may be necessary or desirable to isolate the bottom of the well from the wellhead. Downhole bridge plugs are extensively used in such applications to establish a removable seal in the well.
A conventional downhole bridge plug may include a central mandrel on which is provided at least one expandable sealing element. An annular cone and ridged slip assembly may be provided on the mandrel on each side of the sealing element or elements. The bridge plug may be set in place between adjacent hydrocarbon-producing fractions in the well casing by initially running the bridge plug to the desired location in the casing on a tubing string or using an alternative method and then sliding the slip assemblies onto the respective cones using a hydraulic or other setting tool, causing the slip assemblies to expand against the interior of the casing as they travel on the cones. Simultaneously, the cones move inwardly toward each other and against the sealing rings, causing the rings to expand outwardly against the well casing. Therefore, the slip assemblies and the sealing rings together form a fluid-tight seal to prevent movement of fluids from one fraction to another within the well. When it is desired to re-establish fluid communication between the fractions in the well, the downhole bridge plug may be removed from the well casing. A backup ring may be provided on the mandrel between each cone and the sealing element or elements to reinforce the sealing element or elements after expansion against the casing.
One type of downhole bridge plug, commonly known as a drillable bridge plug, can be removed from the well casing by drilling or milling the bridge plug rather than by retrieving the plug from the casing. In this process, a milling cutter or drill bit is extended through the casing and rotated to grind the plug into fragments until the plug no longer seals the well casing. Drillable bridge plugs may be constructed of a drillable metal, engineering-grade plastic or composite material that can be drilled or ground into fragments by the milling cutter or drill bit.
One drawback of conventional downhole bridge plugs is that the backup rings on the mandrel may inadequately reinforce the sealing element or elements in the casing after the plug expansion process. This may allow the sealing element or elements to slip on the mandrel during application of pressure to the plug. A common drawback of conventional drillable bridge plugs is that during milling or drilling and grinding of the plug, the mandrel has a tendency to rotate or spin with the cutter or drill bit while the sealing rings and other outer sealing components of the plug remain stationary against the interior surface of the well casing. This effect may reduce drilling efficiency and prolong the time which is necessary to remove the plug from the well bore.
Therefore, downhole bridge plug or packer assemblies having slotted expandable backup rings which tightly engage the interior surface of a well casing and reinforce the central sealing element or elements in the plugging of a subterranean well are needed. In some embodiments, the downhole bridge plug assembly may be a drillable bridge plug in which the central mandrel is locked in a stationary configuration with the exterior sealing elements and backup rings of the plug to prevent rotation of the mandrel during removal of the plug from a well casing. In some embodiments, the downhole bridge plug assembly may be a retrievable bridge plug which may be re-usable.